The invention relates to a lateral-duct fan having a lateral duct approximately of the shape of a torus cut normally to its axis of symmetry, and having a coaxial impeller having axially-symmetrically lined-up blades, at its front side, in a ring-shaped recess that essentially faces the lateral duct with a gap located axially therebetween.
A lateral-duct fan of this type is known from the Published German Patent Application No. 21 35 093. Lateral-duct fans or ring-duct fans of this type furnish high pressures, with relatively low efficiency. Their characteristic curves are steep. The output data depend considerably on the adjustment of the axial gap and change in a critical manner with a change of the axial gap.
The present invention, therefore, has for an object the creating of a lateral-duct fan of the initially mentioned type, where the sensitivity of the interdependence between the axial gap and the output is reduced and, with the same axial gaps, the fan output is increased.
This object is achieved, in accordance with a preferred embodiment of the present invention, by the fact that the radius of at least one edge of the ring-shaped recess, measured at a boundary surface between the lateral duct and the impeller, is smaller than the radius of the closest edge of the lateral duct. In this manner, an increase in efficiency (with the same width of the axial gap) and a considerably smaller interdependence between the width of the axial gap and the fan efficiency is surprisingly achieved. The measures taken in accordance with the present invention are based on the following realization: In the case of lateral-duct fans, a helix-shaped secondary flow superimposes itself on the main flow (which travels in a circumferential direction) in such a way that gas from the ring-duct, in the area close to the axis, is drawn into the chambers between the blades of the impeller and is blown out again in the area away from the axis into the ring duct. If, as is usually the case in the known lateral-duct fans, the inside and outside edges of the impeller chambers, i.e., of the ring-shaped recess receiving the blades, are in alignment with the inside and outside edges of the lateral duct, gas coming out of the impeller chamber, because of the existing axial gap, on the outside, rushes against the outside boundary edge of the lateral duct, while vice-versa, in the area close to the axis, the secondary flow leaving the ring duct, rushes against the inside circumferential edge of the impeller chambers. The result is a damming effect which, in an undesirable manner, increases the ineffectiveness of the sealing gap.
Because, according to the present invention, at least one circumferential edge of the impeller chamber is located, by one diametral jump (i.e., a distance corresponding to the difference between the radii of the respective edges) radially within one circumferential edge of the housing ring duct, the damming of gas in the transition area of the secondary flow from the impeller chambers in the housing ring duct and vice-versa are avoided, and the ineffectiveness of the sealing gap is decreased. Because of a reduction of the dynamic pressure achieved by means of the invention, the sensitivity in regard to the interdependence between the axial gap width and the fan output is also reduced.
According to an especially advantageous development of the invention, a diametral jump is not only provided in the area of one edge, but both edges, i.e., at the outside circumferential edge as well as at the inside circumferential edge. The above-described advantageous effect is thus increased even more.
Particularly good results are obtained if, according to a preferred embodiment, the diametral jump at the inside edge and/or the outside edge amounts to 1 to 5 times the dimension of the axial gap.
According to another advantageous aspect of the invention, the lateral duct is constructed in the bottom of an approximately can-shaped housing, in which case the impeller has a cylindrical shape and is disposed in the can-shaped housing in a manner leaving a radial sealing gap. The radial sealing gap should be as narrow and long as possible. By means of this sealing gap, the ineffectiveness or gap less of the axial sealing gap is reduced in the circumferential direction, because the radial gap may partly take over the sealing function of the axial gap and thus increase it. Another increase of the sealing effect and of the rise of the output of the fan can be obtained, according to another advantageous aspect of the invention, by the fact that, in the bottom of the can-shaped housing, a central cylindrical recess is provided into which a corresponding pin projects, that is shaped onto the impeller, while leaving a cylindrical ring gap therebetween. This cylindrical ring gap also promotes the sealing function of the axial gap, as explained above.
The front surface of the impeller, that faces away from the ring or lateral duct, may also be used to further increase the sealing effect, if, according to another advantageous feature, the can-shaped housing is closed by a lid connected with its cylinder jacket, while leaving an axial sealing gap, in the direction of the impeller, in which lid a recess for a drive shaft of the impeller is provided.
Another increase of the sealing effect, according to an advantageous further development of the invention, can be achieved by the fact that the impeller, at its rear surface facing away from the lateral duct, has ribs in the manner of a radial fan, which are located between the impeller and the lid and which, at the back side of the impeller, build up a pressure which counteracts the flow through the cylindrical sealing gap between the can-shaped housing and the impeller.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a single embodiment in accordance with the present invention.